Historically, the treatment of cranial lesions initially proceeded by way of surgical processes—gaining access to brain tissue by way of apertures created in the skull. Clearly, great accuracy was needed in such surgery, and the stereotactic frame was developed for this purpose by Lars Leksell in the late 1940s. This assisted surgeons by providing a precise frame of reference within which to operate. Typically, a stereotactic frame attaches to a patient via pins that extend through the soft anatomy under local anaesthesia and abut the bone to provide a frame of reference that is fixed relative to the rigid body that is the skull. Given that the brain tissue within the skull exhibits relatively little movement during normal movement of the human body, this allowed accurate positioning of surgical instruments relative to the brain tissue and its associated structures.
Leksell then sought to extend the ambit of intracranial surgery to areas that were difficult to reach via surgical methods, such as the base of the skull. To do so, he developed the Gamma Knife, a multi-source radiotherapy apparatus. This comprises 201 Co60 sources mounted on a fixed support, usually hemispherical or cylindrical in form. The sources are distributed about the support, and each is collimated so as to produce a beam that is directed at a single defined point. Thus, the total dose at that point is provided by all the sources, whereas away from that point the total dose is at most that from one or only a small number of sources.
To proceed with treatment via a Gamma Knife, a stereotactic frame is affixed to the patient's bony anatomy in order to fix the position of the patient (and hence the lesion) relative to the device. The volume of lesion to be treated (henceforth referred to as the target) is localized using diagnostic imaging modalities such as computed tomography (CT) and magnetic resonance imaging (MRI) while the frame is in place, and the localization coordinates are then related to the frame. Treatment of the target is then achieved by careful positioning of the frame (with the patient affixed) with respect to the irradiation unit. The frame is therefore a longstanding and essential part of the treatment process for intracranial lesions.